Rates of mental illness among an international force of civilian contractors hired to work in Iraq or Afghanistan rivaled those among service members, a report says.

Rates of mental illness among an international force of civilian contractors hired to work in Iraq or Afghanistan rivaled those among servicemembers, with one in four civilians showing signs of post-traumatic stress disorder, according to a RAND Corp. study released today.

The prevalence of PTSD was even higher among American contractors. Nearly one in three showed signs of the disorder, researchers found in an online survey of 660 civilians working in war zones between early 2011 and early 2013.

“These findings highlight a significant but often overlooked group of people struggling with the after-effects of working in a war zone,” said Molly Dunigan, co-author of the study and a political scientist with RAND.

Researchers said it is unclear how many contractors have served in war zones, but they often outnumber the troops deployed. About 156,000 U.S. contractors, for example, worked with 152,000 U.S. troops in Iraq in 2008; 94,000 Defense Department contractors served with 92,000 U.S. servicemembers in Afghanistan in 2010.

About 260 civilian contractors have died in Iraq and more than 1,375 in Afghanistan, the report says.

“Contractors are often referred to … as a ‘shadow force’ operating below the radar or in the shadows of their military counterparts,” the RAND authors wrote. Nearly two-thirds of those surveyed were Americans.

Signs of PTSD were found among half of those contractors hired for transportation work, which includes the ever-present danger of improvised explosive devices, according to the study.

The vast majority of all contractors went to war zones with health insurance, but about one in five American workers risked injury or death without it. And despite health insurance and access to workers’ compensation for injury or death, relatively few contractors tapped these benefits, the study found.

Many simply avoided doing so out of concern that it would affect their employment, according to the study. Only about 16% of contractors surveyed said they filed workers’ compensation claims.

Among those who showed signs of mental health problems, only 28% sought therapy for PTSD and 34% for depression in the 12 months before taking the survey.

Researchers also found striking statistical differences between the rate of mental and physical health problems among American contractors and those of their peers from Great Britain. More than half of American workers in the war zone reported physical health problems, particularly orthopedic injuries, compared with 16% of British workers.

Researchers found no clear explanation for the difference.

Contractors were older than their uniformed peers in the war zone. About two-thirds were older than 40 and a third were older than 50.